(1) Field of the Invention
The present invention relates to the general field of strain measurement or sensing.
(2) Description of the Art
Conventional strain gauges take a variety of forms and among the most common are resistance strain gauges which exploit the change in resistance of an electrically conductive material when the conductor is stretched or otherwise elastically deformed, and semiconductor strain gauges which exploit the change in the electronic charge across a piezoelectric crystal when the material is mechanically stressed. Such types obviously require the use of associated electronic monitoring equipment in order to give an indication of the measured strain. Purely mechanical strain gauges are also known but generally require precision mechanisms to amplify the small movements sensed and to convert them into an indication of strain. The present invention, on the other hand, seeks to provide a simple and inexpensive form of device capable of giving a visual indication of at least the general level of strain (or a related parameter) in an object to which it is applied, while avoiding the use of electronics and mechanical complexity.
One field of application for the invention with which the applicant is particularly concerned is a device for giving a simple indication of the level of flexural strain in the shaft of a toothbrush. In this respect it is well documented that brushing the teeth too hard can be detrimental to the condition of the teeth and gums. In general relatively low brushing pressure is sufficient to remove plaque from teeth. If teeth are brushed too hard the gums can start to recede causing bleeding of the gums and sensitive teeth. The surface of the teeth can also become damaged irreparably. In this field of application, therefore, an aim is to incorporate a simple visual indicator in a toothbrush which can be seen in a mirror while brushing and will signal if a predetermined strain in the shaft is generated, corresponding to a recommended limit of brushing pressure being exceeded or about to be exceeded.
There are also numerous other kinds of hand-held or hand-operated implement where it could be useful to provide a simple visual indication of strain in some part of the implement e.g. to signal the level of force being applied to a work piece or other object (including the human body) by or through the implement. Examples of such implements include razors, pens or other writing implements and hand tools such as screwdrivers, wrenches and saws etc.
This is, however, but one class of many possible applications for indicators according to the invention, others of which will appear from the ensuing description or will otherwise occur to those skilled in the art and include, without limitation, bridges, buildings and other civil engineering structures, railway tracks, ships' masts, sports rackets and balls, bolts, nuts, pipe couplings, vehicle tyres, fire extinguishers and other pressure vessels, load restraining straps, and other stressed articles.